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United States Patent |
5,141,711
|
Gjertsen
,   et al.
|
August 25, 1992
|
Reconstitutable control assembly having removable control rods with
detachable split upper end plugs
Abstract
A reconstitutable control assembly has a spider structure which supports
control rods by detachable attachment joints. Each attachment joint
includes a hollow connecting finger on the spider structure and an
elongated detachable split upper end plug on each control rod. The split
upper end plug includes a pair of separate upper and lower plug portions.
The upper plug portion has upper, middle and lower sections, whereas the
lower plug portion has upper, middle and lower segments. The upper section
of the upper plug portion is rigidly attached to the connecting finger on
the spider structure, whereas the lower segment of the lower plug portion
is rigidly connected to the control rod. Also, the lower section of the
upper plug portion and the middle segment of the lower plug portion have
complementary threads defined thereon for rigidly threadably attaching the
upper and lower plug portions to one another. Two embodiments of locking
features can be used in conjunction with the upper and lower plug portions
for locking them together so as to resist their unthreading from one
another.
Inventors:
|
Gjertsen; Robert K. (Columbia, SC);
Knott; Ronald P. (Columbia, SC);
Sparrow; James A. (Columbia, SC)
|
Assignee:
|
Westinghouse Electric Corp. (Pittsburgh, PA)
|
Appl. No.:
|
732373 |
Filed:
|
July 18, 1991 |
Current U.S. Class: |
376/327; 376/233 |
Intern'l Class: |
G21C 007/10 |
Field of Search: |
376/327,260,463,440,446,234,233
|
References Cited
U.S. Patent Documents
4189348 | Feb., 1980 | Donck et al. | 376/446.
|
4314885 | Feb., 1982 | Edwards et al. | 376/327.
|
4381283 | Apr., 1983 | Walton | 376/327.
|
4452755 | Jun., 1984 | Hylton | 376/446.
|
4599213 | Jul., 1986 | Leclercq et al. | 376/446.
|
4711756 | Dec., 1987 | Nakazato | 376/327.
|
4820475 | Apr., 1989 | Mayers et al. | 376/327.
|
4855100 | Aug., 1989 | Shallenberger et al. | 376/327.
|
4928291 | May., 1990 | Mouesca et al. | 376/327.
|
Primary Examiner: Behrend; Harvey E.
Parent Case Text
This is a continuation of copending application Ser. No. 07/637,398, filed
Jan. 4, 1991 and now abandoned, which is a divisional of application Ser.
No. 07/426,939, filed Oct. 26, 1989, now U.S. Pat. No. 4,993,864, which is
a divisional of application Ser. No. 07/284,566 filed Dec. 15, 1988, now
U.S. Pat. No. 4,888,151 which is a continuation-in-part of application
Ser. No. 07/232,970, filed Aug. 17, 1988 and now abandoned.
Claims
We claim:
1. In a reconstitutable control assembly for use with a nuclear fuel
assembly, said control assembly including a spider structure and at least
one control rod, an attachment joint for detachably fastening said control
rod to said spider structure, said attachment joint comprising:
(a) a hollow connecting finger on said spider structure;
(b) an elongated detachable split upper end plug on said control rod having
a pair of separate upper and lower plug portions, said upper plug portion
having integrally-connected tandemly-arranged upper, middle and lower
sections, said lower plug portion having integrally-connected
tandemly-arranged upper, middle and lower segments; and
(c) a radially flexible and expandable thread-defining means;
(d) said upper section of said upper plug portion being rigidly attached to
said connecting finger on said spider structure, said lower segment of
said lower plug portion being rigidly connected to said control rod;
(e) said lower section of said upper plug portion and said middle segment
of said lower plug portion being complementarily threaded to receive
therebetween said radially flexible and expandable thread-defining means
for threaded engagement therewith in a relationship in which said
thread-defining means frictionally resists rotation of said upper and
lower plug portions relative to one another;
(f) said middle section of said upper plug portion and said upper segment
of said lower plug portion being complementarily configured for defining a
slip fit joint between said upper and lower plug portions.
2. The attachment joint as recited in claim 1, wherein:
said connecting finger has an axial bore formed in a lower portion thereof
so as to open at a lower end of said finger and a threaded hole at an
inner end of said bore; and
said upper section of said upper plug portion is externally-threaded and
threadably attached within said threaded hole of said connecting finer
axial bore.
3. The attachment joint as recited in claim 1, wherein:
said control rod has an elongated hollow tube with an upper open end; and
said lower segment of said lower plug portion is mated with and rigidly
connected to said upper end of said control rod tube.
4. The attachment joint as recited in claim 1, wherein said complementarily
threaded means defined on said lower section of said upper plug portion
and said middle segment of said lower plug portion are respective external
male threads and internal female threads.
5. The attachment joint as recited in claim 1, wherein said radially
flexible and expandable thread-defining means is in the form of a screw
thread coil insert which threadably fits between and at its respective
interior and exterior peripheries makes threaded frictional engagement
with said external male threads and internal female threads on said
respective upper plug portion lower section and lower plug portion middle
segment.
6. The attachment joint as recited in claim 5, wherein said middle section
of said upper plug portion has a cylindrical surface formed on the
exterior thereof and said upper segment of said lower plug portion is
configured as a cylindrical sleeve for fitting over said cylindrical
surface of said middle section of said upper plug portion for defining a
slip fit joint between said upper and lower plug portions.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
Reference is hereby made to the following copending application dealing
with related subject matter and assigned to the assignee of the present
invention:
"Reconstitutable Control Rod Spider Assembly" by John M. Shallenberger et
al, assigned U.S. Ser. No. 162,841 and filed Mar. 2, 1988, now U.S. Pat.
No. 4,855,100.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to nuclear reactors and, more
particularly, is concerned with a reconstitutable control rod spider
assembly having removable control rods with detachable split upper end
plugs.
2. Description of the Prior Art
In a typical nuclear reactor, such as a pressurized water type, the reactor
core includes a multiplicity of fuel assemblies. Each fuel assembly is
composed of top and bottom nozzles with a plurality of elongated
transversely spaced guide thimbles extending longitudinally between and
attached at opposite ends to the nozzles. Also, a plurality of transverse
support grids are axially spaced along and attached to the guide thimbles.
Further, a plurality of elongated fuel elements or rods transversely
spaced apart from one another and from the guide thimbles are supported by
the transverse grids between the top and bottom nozzles. The fuel rods
each contain fissile material and are grouped together in an array which
is organized so as to provide a neutron flux in the core sufficient to
support a high rate of nuclear fission and thus the release of a large
amount of energy in the form of heat. A liquid coolant is pumped upwardly
through the core in order to extract some of the heat generated in the
core for the production of useful work.
Since the rate of heat generation in the reactor core is proportional to
the nuclear fission rate, and this, in turn, is determined by the neutron
flux in the core, control of heat generation at reactor start-up, during
its operation and at shutdown is achieved by varying the neutron flux.
Generally, this is done by absorbing excess neutrons using control rods
which contain neutron absorbing material. The guide thimbles, in addition
to being structural elements of the fuel assembly, also provide channels
for insertion of the neutron absorber control rods within the reactor
core. The level of neutron flux and thus the heat output of the core is
normally regulated by the movement of the control rods into and from the
guide thimbles.
One common arrangement utilizing control rods in association with a fuel
assembly can be seen in Hill U.S. Pat. No. 4,326,919. This patent shows a
control rod spider assembly which includes a plurality of control rods and
a spider structure supporting the control rods at their upper ends. The
spider structure, in turn, is connected to a control drive mechanism that
vertically raises and lowers (referred to as a stepping action) the
control rods into and out of the hollow guide thimbles of the fuel
assembly. The typical construction of the control rod used in such an
arrangement is in the form of an elongated metallic cladding tube having a
neutron absorbing material disposed within the tube and with end plugs at
opposite ends thereof for sealing the absorber material within the tube.
The spider structure typically includes a plurality of radially extending
vanes supported on and circumferentially spaced about a central hub. The
vanes are flat metal plates positioned on edge and being connected at
their inner ends to the central hub. Cylindrical shaped control rod
connecting fingers are mounted to and supported by the vanes, with some of
the vanes having only a single connecting finger and other vanes having a
spaced pair of connecting fingers associated therewith.
Typically, the upper end plug of each control rod has a threaded outer end
which is receivable into a bore in the lower portion of one finger of the
spider structure and threadable into a tapped hole formed in the finger at
the inner end of the bore. The end plug is then secured or locked therein
by a key or pin inserted into the side of the finger and the end plug and
then welded therein. Generally speaking, the above-described control rod
spider assembly is not reconstitutable, that is, the assembly cannot
readily be taken apart and worn or damaged components replaced. Instead,
the whole assembly must be discarded.
In many older nuclear reactor plants, the control rods are approaching the
end of their design life. Furthermore, in a number of newer plants with
modified reactor internals, the control rods are experiencing accelerated
wear. In most cases wear occurs over local areas on the cladding of some,
but not all, control rods of a given control rod spider assembly. Wear is
believed to be caused by coolant flow-induced vibration. The severity of
the clad wear is dependent upon the type of nuclear plant and internals
design and typically occurs only on a limited number of control rods of a
given assembly.
Consequently, there is a growing interest in and need for reconstitutable
control rod spider assemblies wherein selected individual control rods
that are prematurely worn or damaged can be removed and replaced. A spider
assembly which allows removal of a control rod from a spider structure by
severing or cutting off the threadably fastened top end of the control rod
and the fastener is disclosed in French patent application No. 86/08381. A
spider assembly which seems to allow removal of a control rod from a
spider by removing a set screw and unthreading the upper end plug of the
control rod from a support rod axially movably mounted to a spider vane
finger is disclosed in Nakazato U.S. Pat. No. 4,711,756. Spider assemblies
which allow removal of a control rod from a spider structure by deforming
a locking element on the top end of the control rod are disclosed in
Edwards et al U.S. Pat. No. 4,314,885 and Walton U.S. Pat. No. 4,281,283.
However, none of these disclosures would appear to be the optimum approach
to providing a reconstitutable control rod spider assembly.
SUMMARY OF THE INVENTION
The present invention provides a reconstitutable control rod spider
assembly designed to satisfy the aforementioned needs. The reconstitutable
control assembly of the present invention incorporates a removable control
rod with a split upper end plug for detachably attaching the control rod
to a vane finger of the spider. Specifically, the split upper end plug is
composed of two separate upper and lower portions.
The upper and lower portions of each upper end plug are attached together
by screw threading and are torqued to incorporate joint preload. Also, a
locking feature is defined by the portions in order to prevent their
loosening during reactor operation. Locking is accomplished by
swaging/crimping a thin-walled upper segment of the lower portion onto a
flats-bearing middle section of the lower portion of the upper end plug.
The flats are thus provided on the upper portion to prevent subsequent
joint loosening. Removal of a control rod is accomplished by unscrewing
the rod which will rotate the lower portion of the upper end plug relative
to the upper portion thereof and overcome the crimp retainer locking
feature.
A replacement control rod can then be reinstalled and crimped for rod
retention. The thin-walled upper section to be crimped is virgin material
on the lower portion of the upper end plug on the replacement control rod.
The detachable attachment features incorporated by the separate upper and
lower portions of the upper end plug do not adversely impact an existing
desired feature such as the flexure joint which is retained by the control
rod upper end plug.
As an alternative to above-described locking feature, a radially flexible
and expandable thread-defining coil can be employed between the threaded
lower section and middle segment of the respective upper and lower end
plug portions to provide frictional resistance to rotation once attachment
of the portions has been completed. The coil makes it possible to rotate
the control rod against the frictional resistance posed by the coil
without the need for overriding or overcoming a crimp retainer-type
locking feature. Where the crimp locking feature was previously employed,
the middle section and upper segment of the respective upper and lower end
plug portions are of cylindrical configuration and form a slip fit type
joint instead.
Accordingly, the present invention is directed to an end plug for use in
facilitating replacement of a neutron absorber control rod on a control
assembly spider structure. The end plug comprises a pair of separate upper
and lower plug portions with the upper plug portion having integral upper,
middle and lower sections and the lower plug portion having integral
upper, middle and lower segments. The upper section of the upper plug
portion is configured for rigid attachment to the control assembly spider
structure for supporting the control rod. The lower segment of the lower
plug portion is configured for rigid connection to the control rod. Also,
the lower section of the upper plug portion and the middle segment of the
lower plug portion are complementarily configured for rigid threaded
attachment to one another. Further, the middle section of the upper plug
portion and the upper segment of the lower plug portion are
complementarily configured for interlocking attachment to one another so
as to resist unthreaded detachment of the upper and lower plug portions
from one another.
Also, the present invention sets forth in a reconstitutable control
assembly including a spider structure and at least one control rod, an
attachment joint for detachably fastening the control rod to the spider
structure. The attachment joint comprises: (a) a hollow connecting finger
on the spider structure; and (b) an elongated detachable split upper end
plug on the control rod having a pair of separate upper and lower plug
portions. The upper plug portion has integrally-connected
tandemly-arranged upper, middle and lower sections. The lower plug portion
has integrally-connected tandemly-arranged upper, middle and lower
segments.
More particularly, the upper section of the upper plug portion is rigidly
attached to the connecting finger on the spider structure, whereas the
lower segment of the lower plug portion is rigidly connected to the
control rod. Also, the lower section of the upper plug portion and the
middle segment of the lower plug portion have complementarily threaded
means defined thereon for rigidly threadably attaching the upper and lower
plug portions to one another. Further, the middle section of the upper
plug portion and the upper segment of the lower plug portion have
complementary interlocking means defined thereon for locking the upper and
lower plug portions together so as to resist unthreading of the upper and
lower plug portions from one another.
Further, the present invention is directed to a modified end plug for use
in facilitating replacement of a neutron absorber control rod on a control
assembly spider structure. The modified end plug comprises a pair of
separate upper and lower plug portions with the upper plug portion having
integral upper, middle and lower sections and the lower plug portion
having integral upper, middle and lower segments. Also, the end plug
includes means in the form of a radially flexible and expandable
thread-defining coil.
Further, the upper section of the upper plug portion is configured for
rigid attachment to a control assembly spider structure for supporting a
control rod, whereas the lower segment of the lower plug portion is
configured for rigid connection to the control rod. Still further, the
lower section of the upper plug portion and the middle segment of the
lower plug portion are threaded complementarily to receive therebetween
the radially flexible and expandable thread-defining coil for threaded
engagement therewith in a relationship in which the thread-defining coil
frictionally resists rotation of the upper and lower plug portions
relative to one another. Finally, the middle section of the upper plug
portion and the upper segment of the lower plug portion are configured
complementarily for defining a slip fit joint between the upper and lower
plug portions.
These and other features and advantages of the present invention will
become apparent to those skilled in the art upon a reading of the
following detailed description when taken in conjunction with the drawings
wherein there is shown and described an illustrative embodiment of the
invention.
BRIEF DESCRIPTION OF THE DRAWINGS
In the course of the following detailed description, reference will be made
to the attached drawings in which:
FIG. 1 is a side elevational view, with parts partially sectioned and
broken away for purposes of clarity, of a prior art fuel assembly having a
prior art control rod spider assembly disposed above the fuel assembly.
FIG. 2 is a top plan view of the control rod spider assembly removed from
the fuel assembly of FIG. 1.
FIG. 3 is an elevational view, with parts partially sectioned, of the
control assembly as seen along line 3--3 of FIG. 2.
FIG. 4 is an enlarged fragmentary view, with parts partially sectioned and
broken away, of a spider structure of the control assembly of FIG. 3.
FIG. 5 is a cross-sectional fragmentary view of a vane and control rod
connecting finger of the spider structure taken along line 5--5 of FIG. 4.
FIG. 6 is a cross-sectional view of the connecting finger taken along line
6--6 of FIG. 4.
FIG. 7 is an enlarged fragmentary view of one control rod of the control
assembly of FIG. 3, illustrating the upper end plug thereof.
FIG. 8 is an enlarged fragmentary view, with parts partially sectioned and
broken away, of the control assembly of FIG. 3, illustrating a prior art
attachment joint between the control rod connecting finger on one vane of
the spider structure of the control assembly and the upper end plug of one
of the control rods thereof.
FIG. 9 is a cross-sectional view of the upper end plug inserted in the
connecting finger taken along line 9--9 of FIG. 8, illustrating a pin
locking the end plug and finger together.
FIG. 10 is a view similar to that of FIG. 8, but illustrating a detachable
split upper end plug on a control rod being constructed in accordance with
the principles of the present invention.
FIG. 11 is an exploded view, on a slightly smaller scale than in FIG. 10,
of the detachable split upper end plug by itself.
FIG. 12 is an enlarged bottom plan view of an upper plug portion of the
split upper end plug as seen along line 12--12 of FIG. 11.
FIG. 13 is an enlarged top plan view of a lower plug portion of the split
upper end plug as seen along line 13--13 of FIG. 11.
FIG. 14 is an enlarged fragmentary side elevational view of the upper plug
portion of the split upper end plug of FIG. 11.
FIG. 15 is an enlarged fragmentary longitudinal axial sectional view of the
lower plug portion of the split upper end plug of FIG. 11.
FIG. 16 is an enlarged fragmentary side elevational view, partly in
longitudinal section, of the split upper end plug of FIG. 10 illustrating
the upper and lower plug portions attached together.
FIG. 17 is a view similar to that of FIG. 10, but illustrating a modified
form of the detachable split upper end plug which employs a different type
of locking feature.
FIG. 18 is a view similar to that of FIG. 11, but illustrating the modified
split upper end plug.
FIG. 19 is a view similar to that of FIG. 14, but illustrating the upper
plug portion of the modified split upper end plug.
FIG. 20 is a view similar to that of FIG. 15, but illustrating the lower
plug portion of the modified split upper end plug.
FIG. 21 is a view similar to that of FIG. 16, but illustrating the upper
and lower plug portions of the modified split upper end plug attached
together.
FIG. 22 is an enlarged side elevational view of a radially flexible and
expandable thread-defining member incorporated in the modified split upper
end plug of FIG. 21.
FIG. 23 is a fragmentary plan view of the member as seen along line 23--23
of FIG. 22.
DETAILED DESCRIPTION OF THE INVENTION
In the following description, like references characters designate like or
corresponding parts throughout the several views. Also in the following
description, it is to be understood that such terms as "forward",
"rearward", "left", "right", "upwardly", "downwardly", and the like, are
words of convenience and are not to be construed as limiting terms.
In General
Referring now to the drawings, and particularly to FIG. 1, there is shown
an elevational view of a nuclear reactor fuel assembly, represented in
vertically foreshortened form and being generally designated by the
numeral 10. Being the type used in PWR, the fuel assembly 10 basically
includes a lower end structure or bottom nozzle 12 for supporting the
assembly on the lower core plate (not shown) in the core region of a
reactor (not shown), and a number of longitudinally extending guide tubes
or thimbles 14 which project upwardly from the bottom nozzle 12. The
assembly 10 further includes a plurality of transverse grids 16 axially
spaced along the guide thimbles 14 and an organized array of elongated
fuel rods 18 transversely spaced and supported by the grids 16. Also, the
assembly 10 has an instrumentation tube 20 located in the center thereof
and an upper end structure or top nozzle 22 removably attached to the
upper ends of the guide thimbles 14 to form an integral assembly capable
of being conveniently handled without damaging the assembly parts.
As mentioned above, the fuel rods 18 in the array thereof in the assembly
10 are held in spaced relationship with one another by the grids 16 spaced
along the fuel assembly length. Each fuel rod 18 includes nuclear fuel
pellets 24 and the opposite ends of the rod are closed by upper and lower
end plugs 26,28 to hermetically seal the rod. Commonly, a plenum spring 30
is disposed between the upper end plug 26 and the pellets 24 to maintain
the pellets in a tight, stacked relationship within the rod 18. The fuel
pellets 24 composed of fissile material are responsible for creating the
reactor power of the nuclear reactor. A liquid moderator/coolant such as
water, or water containing boron, is pumped upwardly through the fuel
assemblies of the core in order to extract heat generated therein for the
production of useful work.
Prior Art Control Rod Spider Assemblies
Turning now to FIGS. 2 and 3 as well as FIG. 1, there is shown a typical
embodiment of a conventional control rod spider assembly, generally
designated 32, designed for use in the conventional fuel assembly 10 of
FIG. 1. In its basic components, the control assembly 32 includes a
plurality of control rods 34 and a spider structure 36 which supports the
control rods at their upper ends. The spider structure 36 holds the
control rods 34 in a pattern matched to that of the guide thimbles 14
which adapts them to be inserted through the top nozzle 22 and downward
through the guide thimbles 14 of the PWR fuel assembly 10. The spider
structure 36 is connected to a control mechanism (not shown) which is
operable in a known manner to move the control rods 34 so as to regulate
core power.
In a typical construction, each control rod 34 of the control assembly 32
is composed of an elongated metallic cladding tube 38 having a neutron
absorbing material disposed therein and upper and lower end plugs 40,42
attached at opposite ends of the cladding tube 38 for sealing the absorber
material therewithin. The spider structure 36 of the control assembly 10
typically includes a plurality of radially extending flukes or vanes 44
supported on and circumferentially spaced about a central hub 46.
Cylindrical shaped control rod connecting fingers 48 are mounted to and
supported by the vanes 44. Some of the vanes 44 have only a single
connecting finger 48 attached thereon, whereas other vanes 44 have a
spaced pair of connecting fingers 48 associated therewith.
Turning now to FIGS. 4-9, there is illustrated one prior art attachment
joint, generally indicated by the numeral 50, provided between each
control rod connecting finger 48 on the vane 44 of the control assembly
spider structure 36 and the upper end plug 40 of each control rod 34.
Typically, the upper end plug 40 of each control rod 34 has a threaded
outer end 52. Each connecting finger 48 is mounted to the vane 44 in a
bayonet-type of welded connection, and has an axial bore 54 formed in a
lower portion 56 thereof with a smaller-diameter threaded hole 58 tapped
therein at the inner end of the bore 54. The threaded outer end 52 of the
upper end plug 40 is threadably received in the tapped hole 58 when the
plug 40 is received within the axial bore 54. The end plug 40 is secured
or locked therein by a key or pin 60 inserted through aligned holes 62,64
in the sides of the finger 48 and end plug 40 and then welded thereto.
Parenthetically, it should be pointed out that the axial bore 54
terminates at the start of an upper portion 66 of each connecting finger
48 where the finger connects with the vane 44.
A major disadvantage of this conventional control assembly 32 is that it is
not reconstitutable; that is, the assembly 32 cannot readily be taken
apart and have worn or damaged components thereof replaced. Instead, the
whole assembly has to be discarded.
However, as mentioned earlier, control rod spider assemblies having
removable control rods are known in the prior art. One recent control rod
spider assembly that is reconstitutable is disclosed in the aforecited
French patent application No. 86/08381. Similar to the above-described
prior art control assembly, the French control assembly includes a spider
structure with connecting fingers on vanes and a plurality of control rods
with upper end plugs having a threaded outer end. However, the attachment
joint employed to secure each control rod to one connecting finger is
modified somewhat from that described above. Each connecting finger of the
French control assembly has an axial bore extending therethrough from end
to end. When the upper end plug of one control rod is inserted through the
axial bore, its threaded outer end extends above the top end of the
finger. A fastener or nut is threaded onto the outer end of the control
rod upper end plug until it contacts the top end of the finger. Then a
tubular locking cup formed on the control rod upper end plug above the
threaded outer end and extending above the threaded nut is deformed
radially outward to lock within a groove in the nut to retain the nut
thereon.
However disadvantageously, to remove the control rod from the spider
structure, the portion of the end plug which includes the threaded outer
end with the nut fastened thereon must first be severed or cut off. Thus,
the fastening nut must be replaced after removal.
Control Rod Spider Assembly of the Present Invention
Turning now to FIGS. 10-16, there is shown particularly in FIG. 10, in
fragmentary form, a reconstitutable control rod spider assembly, generally
designated by the numeral 68 and constructed in accordance with the
principles of the present invention, which provides an improved
arrangement for removing and replacing a control rod. The reconstitutable
control assembly 68 basically includes a spider structure 70, a plurality
of control rods 72 (only one fragmentarily shown), and a plurality of
improved attachment joints 74 (only one shown) for detachable fastening
the control rods 72 to the spider structure 70. Except for the parts
thereof to be described hereinafter, the spider structure 70 and control
rods 72 of the control assembly 68 are identical to that of the prior art
control assembly 32 of FIGS. 2 and 3.
Each attachment joint 74 includes an elongated connecting finger 76 on the
spider structure 70 and an elongated detachable split upper end plug 78 on
each control rod 72. The split upper end plug 78 includes a pair of
separate upper and lower plug portions 80,82. The upper plug portion 80
has integrally-connected tandemly-arranged upper, middle and lower
sections 80A,80B,80C, whereas the lower plug portion 82 has
integrally-connected tandemly-arranged upper, middle and lower segments
82A,82B,82C.
Being substantially identical to the prior art attachment joint 50 of FIG.
8, the connecting finger 76 of the attachment joint 74 of the present
invention has an axial bore 84 formed in a lower portion 76A of the finger
76 so as to open at a lower end 86 of the finger, and an
internally-threaded hole 88 at an upper end of the bore 84. Further, the
upper section 80A of the upper plug portion 80 has external threads 90
allowing it to be rigidly threadably attached within the threaded hole 88
of the connecting finger 76. Also, the lower segment 82C of the lower plug
portion 82 is of a smaller diameter size than an upper open end 92 of an
elongated hollow tube 94 of the control rod 72 allowing it to be mated and
rigidly connected within the control rod tube upper end 92. However, the
remaining middle and lower sections 80B,80C of the upper plug portion 80
and the upper and middle segments 82A,82B of the lower plug portion 82 are
different from the upper end plug 40 of the prior art attachment joint 50
of FIG. 8.
More particularly, in the attachment joint 74 of the present invention, the
lower section 80C of the upper plug portion 80 and the middle segment 82B
of the lower plug portion 82 have complementarily threaded means 96,98
defined thereon for rigidly threadably attaching the upper and lower plug
portions 80,82 of the split upper end plug 78 to one another. The
complementarily threaded means 96 defined on the lower section 80C of the
upper plug portion 80 is preferably in the form of external male threads
96. The complementarily threaded means 98 on the middle segment 82B of the
lower plug portion 82 preferably includes internal female threads defined
in a recess 100 at the inner end of a central opening 102 extending
through the upper and middle segments 82A,82B of the lower plug portion
82. The external threads 96 of the upper plug portion 80 are threadably
received within the internal threads 98 of the lower plug portion 82 when
the portions are attached together.
Further, lower section 80C of the upper plug portion 80 has two different
diametric sizes. At the smaller one of the two diameters of the lower
section 80C is defined the external threads 96. Bridging the two diameter
sizes of the lower section 80C is an annular shoulder 104 which faces
toward the lower plug portion 82 and is located just above the external
threads 96. The central opening 102 of the middle segment 82B of the lower
plug portion 82 is larger in diameter than the internally threaded recess
100 thereof so as to define therebetween an annular ledge 106 located just
above the recess 100. The upper plug portion shoulder 104 is capable of
being bottomed out in a preloaded condition against the lower plug portion
ledge 106 when the upper and lower plug portions 80,82 are threadably
attached together.
Further, in the attachment joint 74 of the present invention, the middle
section 80B of the upper plug portion 80 and the upper segment 82A of the
lower plug portion 82 have complementary interlocking means 108,110
defined thereon for locking the threadably attached upper and lower plug
portions 80,82 together so as to resist unthreading thereof from one
another during normal reactor operation. The complementary interlocking
means 108 defined on the middle section 80B of the upper plug portion 80
is a plurality of flat surfaces 108 formed on the exterior thereof at
locations circumferentially spaced from one another. There are preferably
four flat surfaces 108 being circumferentially displaced about ninety
degrees from one another. The complementary interlocking means 110 defined
on the upper segment 82A of the lower plug portion 82 is a hollow sleeve
110 fitted over and about the flat surfaces 108 and deformably crimped
thereagainst so as to resist unthreading of the upper and lower plug
portions 80,82 from one another. However, the sleeve 110 is thin-walled so
as to reformable back to a generally cylindrical configuration upon
application of a predetermined torque to the control rod 72 to rotate the
same and unthread the lower and upper plug portions 80,82 of the split
upper end plug 78 from one another. As can be seem in FIGS. 10, 15 and 16,
the upper segment 82A of the lower plug portion 82 defining the sleeve 110
is much thinner in wall thickness than the middle segment 82B thereof.
To summarize, the reconstitutable control assembly 68 of the present
invention incorporates a plurality of removable control rods 72 each with
a split upper end plug 78 for detachably attaching the control rod 72 to a
vane finger 76 of the spider structure 36. Specifically, the split upper
end plug 78 is composed of two separate upper and lower plug portions
80,82. The upper and lower plug portions 80,82 of each upper end plug 78
are attached together by screw threading and are torqued to incorporate
joint preload. Also, the interlocking flat surfaces 108 and crimped sleeve
110 prevent loosening of the plug portions 80,82 during reactor operation.
Locking is accomplished by swaging/crimping the thin-walled sleeve 100 of
the lower plug portion upper segment 82A onto flat surfaces 108 of the
upper plug portion middle section 80B. The flat surfaces 108 are thus
provided on the upper plug portion to prevent subsequent joint loosening.
Removal of a control rod 72 is accomplished by unscrewing the rod which
will rotate the lower plug portion 82 of the upper end plug 78 being
fixedly attached to the control rod tube 94 relative to the upper plug
portion 80 thereof being fixedly attached to the spider vane connecting
finger 79 and overcome the crimp retainer locking feature provided by the
deformed sleeve 110. A replacement control rod can then be reinstalled and
crimped for rod retention. The replacement thin-walled sleeve to be
crimped is virgin material on the lower plug portion of the upper end plug
on the replacement control rod. The detachable attachment features
incorporated by the separate upper and lower plug portions 80,82 of the
split upper end plug 78 do not adversely impact any existing desirable
features, such as the flexure joint 112 on the upper plug portion 80 and
the intended clearance gap 114 between the flexure joint 112 and the lower
end 86 of the finger 76; these prior art features are retained in the
split upper end plug 78 of the present invention.
Turning now to FIGS. 17-21, a modified split upper end plug 116 is
illustrated having upper and lower plug portions 118,120 and employing a
different type of locking feature as an alternative to the above-described
crimp retainer-type locking feature. In all respects but the following
ones the modified split upper end plug 116 is identical to the
earlier-described split upper end plug 78.
First, in place of the thin-walled crimpable hollow sleeve 110 of the upper
section 82A of the upper end plug lower portion 82 and the flats-bearing
middle segment 80B of the upper end plug upper portion 80, a
thicker-walled cylindrical upper segment 120A is provided on the lower
plug portion 120 and a cylindrical nonflat-bearing middle section 118B is
provided on the upper plug portion 118. Thus, where the crimp locking
feature was previously employed, the corresponding middle section 118B and
upper segment 120A of the respective upper end plug portions 118,120 are
of cylindrical configuration and merely form a slip fit type joint
instead.
Second, the threaded lower section 118C of the upper plug portion 118 and
threaded middle segment 120B of the lower plug portion 120 of the modified
split upper end plug 116 are greater in length than the corresponding
threaded lower section 80C and middle segment 82B of the respective upper
and lower portions 80,82 of the split upper end plug 78. Finally, the
modified locking feature is provided by a member 122 in the form of a
radially flexible and expandable thread-defining coil deployed between the
threaded lower section and middle segment 118C,120B of the respective
upper and lower end plug portions 118,120. More particularly, the coil 122
threadably fits between and at its respective interior and exterior
peripheries makes threaded frictional engagement with external male
threads and internal female threads 124,126 on the respective upper plug
portion lower section 118C and lower plug portion middle segment 120B.
The coil 122 thus provides frictional resistance to relative rotation of
the upper and lower plug portions 118,120 once attachment of the plug
portions has been completed. However, the coil makes it possible to rotate
the control rod against the frictional resistance posed by the coil
without the need for overriding or overcoming a crimp retainer-type
locking feature.
As best seen in FIGS. 22 and 23, the radially flexible and expandable
thread-defining coil 122 is a screw thread coil insert which per se is a
commercial product marketed under the tradename HeliCoil. The locking
action of the coil is achieved by one or more of the windings 128 of the
coil 122 having a series of straight segments or "chords" 128A. When the
threaded lower section 118C of the upper plug portion 118 enters the
"grip" coil 122, these chordal segments 128A flex outward, creating
pressure on the threaded section and providing frictional resistance to
rotation of the upper plug portion 118 relative to the lower plug portion
120.
It is thought that the present invention and many of its attendant
advantages will be understood from the foregoing description and it will
be apparent that various changes may be made in the form, construction and
arrangement thereof without departing from the spirit and scope of the
invention or sacrificing all of its material advantages, the form
hereinbefore described being merely a preferred or exemplary embodiment
thereof.
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